A single-phase A.C. is usually used as a power supply of such as a spot welding equipment, a lighting equipment, an electric motor, or an electric heater. To obtain the single-phase A.C., a device for converting a three-phase A.C. to a single-phase A.C. such as a Scott wiring, a three-phase low frequency system, and an inverter system is used, but in case of using the device, the unbalance between three phases occurs as the current increases and the efficiency of the electric power is low. The above-described power supply unit such as Scott wiring has a complicated circuit construction and a large size and it cannot take out a stable current because a twice excessive current flows in one of three phases. As well, in case of using the three-phase low frequency system as a power supply unit of the spot welding equipment, the equipment is of large-sized, expensive, and likely to have frequent problems.
Since the arc welding equipment generally requires a large amount of current, two kinds of power supply systems, that is, a magnetic leakage system and a reactor system are employed therefor. The former utilizes a magnetic leakage transformer as a transformer and the latter is one constituted by inserting a saturable reactor in series in a discharging circuit comprising a secondary side of a transformer and an arc electrode. These systems can provide an output having a drooping characteristic that the value of the output voltage abruptly rises up to a high voltage and thereafter drops swiftly as in arc welding, but the magnetic leakage and the reactance of the reactor cause a large loss in the output.
One of recently adopted systems is an inverter system that rectifies an A.C. and raises the frequency of the A.C. from about several hundred cycles to about 1200 cycles, and then inputs the A.C. to a transformer and again rectifies the output of the transformer so as to obtain a D.C. as an output of a secondary side. The use of the inverter system enables to make the transformer of small-sized and lightweight but the system is extremely expensive as well as of bad efficiency and likely to have frequent troubles. Since a single-phase A.C. power supply unit of 200 volts is used as a power supply in any system, the secondary side output is apt to be affected by pulsation of the A.C., and especially it fluctuates in the neighborhood of the zero point in the sinusoidal wave of the A.C. Therefore, the arc welding equipment requires keeping a proper distance between a welding rod and a weldment to suppress arcs, and the welding operation such as travel speed control requires a great deal of skill. Therefore, such welding equipment requires a small-sized and lightweight, and high efficiency power supply unit.
In addition, as shown in FIG. 6, the conventional multi-arc welding equipment includes a three-phase transformer 16 having a primary and a secondary coil 28 and 29 and applies three-phase output Generated at the secondary coil 29 to the three welding electrodes to generate multiple-arcs between the welding electrodes and the weldment and between welding electrodes for carrying out the multiple-arc (hereinafter referred to as multi-arc) welding.